Apparatus and method using continuous-wave radiation for detecting and locating targets hidden behind a surface

ABSTRACT

Continuous-wave radiation is used to detect a target hidden behind a surface. In an embodiment, a transmitter directs a beam of continuous-wave microwave radiation from a transmitting location, and reflected radiation from the target is received at first and second receiving locations closer to the surface than the transmitting location. The transmitting and receiving locations have spatial relationships such that the phase of reflected radiation received at one receiving location is in quadrature with the phase of reflected radiation received at the other receiving location. In an embodiment, direct transmitted radiation is received at the receiving locations in quadrature.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of provisional Application No.60/413,757 filed Sep. 27, 2002, and provisional Application No.60/474,962 filed Jun. 3, 2003, both incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention is concerned with the detection and location of targetshidden behind a surface, such as a surface of the earth, usingcontinuous-wave radiation.

Although the detailed description of the invention refers to measurementof reflected electromagnetic radiation at microwave wavelengths, theprinciples of the invention are applicable to other types ofcontinuous-wave systems, such as those using sound waves (e.g., sonar).

Various means and methods have been developed for detection and locationof buried metallic and non-metallic objects which rely on thereflectivity of the objects at radar (microwave) wavelengths. Thesemeans and methods include devices which attempt to image the objectsthrough beam scanning and determine distance (range) by timingdifferences between transmitted and reflected pulses (i.e.,ground-penetrating radars) as well as devices which attempt to utilizeexpected differences between background (earth) reflectivity and thereflectivity of the buried object. Both methods are subject tosignificant difficulties in their ability to locate buried objects(especially non-metallic objects) due to several factors. These include:

(a) presence of other buried materials in surrounding soil (rocks, treeroots, etc.) whose reflectivities are comparable to that of the targetobject;

(b) rough or uneven terrain surface which produces widely-varyingbackground reflected signals;

(c) for continuous-wave devices, constructive and destructiveinterference between transmitted and reflected waves;

(d) interference between multi-path reflected signals; and

(e) interference between the fundamental frequency and harmonics in thereflected wave.

Although pulsed devices which rely on timing are less subject tointerference problems than continuous-wave devices, continuous-wavedevices are inherently less complex, require less power, and may be mademore easily portable.

BRIEF DESCRIPTION OF THE INVENTION

The following description relates to a continuous-wave device comprisinga transmitter and two or more receivers designed to detect and locateburied metallic and non-metallic objects by measurement of reflectedmicrowave radiation, and discloses the means and methods used toovercome or diminish some of the difficulties described above.

The invention will be described with reference to two embodiments whichare designed to detect targets beneath the surface of the earth, but itwill become apparent in later portions of the description that theinvention is useful in detecting targets hidden behind wall surfaces,for example.

Both embodiments of the invention use a transmitter that transmits abeam of continuous-wave radiation and a pair of receivers of suchradiation. Predetermined spatial relationships (geometry) of thetransmitter and the receivers are provided such that the transmitter isfarther from the surface than the receivers and such that a quadraturephase relationship exists for reflected radiation at the receivers. Inone embodiment, a quadrature relationship also exists for directradiation that reaches the receivers from the transmitter.

Although not so restricted, in both embodiments the transmitter and thereceivers are mounted on an elongated hand-held rod, with the receiversadjacent to an end of the rod and the transmitter farther from the endof the rod than the receivers. For microwave applications of theinvention, directional antennas are used at the transmitter and each ofthe receivers. In one embodiment, the axis of each beam pattern is alongthe length of the rod. In another embodiment, parallel axes of thereceiver beam patterns are inclined with respect to the length of therod, and the axis of the transmitter bean pattern is also inclined withrespect to the length of the rod, but at a different angle ofinclination than that for the receivers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described in conjunction with theaccompanying drawings, which illustrate preferred (best mode)embodiments, and wherein:

FIG. 1 is a view showing a first embodiment of the invention in use;

FIG. 2 is a perspective view of the first embodiment;

FIG. 3 is a fragmentary perspective view showing a portion of the firstembodiment;

FIG. 4 is a fragmentary perspective view showing another portion of thefirst embodiment;

FIG. 5 is a plan view of an antenna that can be used in the invention;

FIG. 6 is a graphical view showing the results of an actual test of thefirst embodiment;

FIG. 7 is another graphical view showing test results of the firstembodiment;

FIG. 8 is a diagram showing spatial relationships employed in the firstembodiment;

FIG. 9 is a view showing a second embodiment of the invention in use;

FIG. 10 is a fragmentary perspective view of the second embodiment;

FIG. 11 is a fragmentary elevation view showing a portion of the secondembodiment;

FIG. 12 is a perspective view of the second embodiment;

FIG. 13 is diagram showing spatial relationships employed in the secondembodiment;

FIG. 14 is another diagram showing spatial relationships employed in thesecond embodiment; and

FIGS. 15A and 15B constitute a block diagram showing a circuit that canbe used in the second embodiment.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

FIGS. 1-5 show apparatus 10 employed in the first embodiment. In thisapparatus, there are three antennas, namely, a transmitter antenna XMMand two receiver antennas RCV1 and RCV2. In the later description, thereceiver antenna RCV2 referred to as the “top antenna” is actuallycloser to the ground than the “bottom antenna” RCV1 in the use of theapparatus. All three antennas are mounted on a rod 12, such as a 1-inchsquare fiberglass tube. Basswood blocks 14 are used to attach theantennas to the tube.

The two receiver antennas are mounted on opposite 4 sides of the rod 12,and in use the top antenna RCV2 is 0.6 inch closer to the ground thanthe bottom antenna RCV1. The transmitter antenna XMTR is mounted on thesame side of the rod as the top antenna RCV2 and is at equal distancesfrom the receiver antennas. For example, the transmitter antenna may beten inches from the receiver antennas. This distance is not critical,but should be the same for both receiver antennas.

In the apparatus shown, each antenna is a directional log periodicantenna having the gain of a Yagi but in a smaller space. See FIG. 5.The antenna elements 16 and 18 are mounted on opposite sides of a PCboard 20. The elements on one side of the board are tied to the elementson the opposite side of the board, which are fed with the centerconductor of a coaxial cable 22.

As shown in FIG. 2, the rod is angulated and has a handle 24 at one endthat is held by an operator when the apparatus is in use as shown inFIG. 1. Mounted on the rod adjacent to the handle is an electronics unit26 that includes a source of continuous microwave energy, a batterypower supply, one or more readout devices (e.g., visual and/or audible)and various controls (e.g., background null and gain adjust). Typicalcircuitry for use in the invention will be described later in connectionwith the second embodiment.

In the use of the apparatus, as shown in FIG. 1, a beam of radiation istransmitted into the ground toward a hidden target and radiationreflected from the target is received by the receivers and produces anoutput.

Continuous-Wave Transmitter/Receiver Design

Consideration of factors (a) through (e) above leads to a number ofconstraints in the design of a continuous-wave detector. Factors (a) and(b) lead to a requirement that the transmitter be situated high enoughabove the terrain surface that, for a reasonably restricted transmitterbeamwidth, the transmitter beam will illuminate an area of the groundthat is large compared to the size of the non-target irregularities(rocks, etc.) and terrain roughness scale. The same considerations applyto the height requirement for the receiving antenna(s) subject to thetwo additional requirements that the receiving antenna(s) must be asclose as possible to the ground for maximum sensitivity to the reflectedsignal from the target, and as far as possible from the transmitter toreduce the direct signal.

Factor (c) above suggests that a pair of receiving antennas separated bysome appropriate fraction of a wavelength might be used to insure thatboth receivers can never simultaneously be located at an interferencenull, while (d) and (e) impact the receiver antenna's beamwidth patternand tuning characteristics, respectively. Analysis of wave pathlengthsand phases indicates that the transmitter antenna must lie above thepair of receiver antennas for the reduction of interference effects.

FIG. 8 shows spatial relationships of the transmitter and the receiversof the first embodiment, and more particularly shows path lengths andphase differences, as follows:

DIRECT PATHS: RCV1: S1=a

RCV2: S2=a+b

REFLECTED PATHS: RCV1: S1′=Rx+R1

RCV2: S2′=Rx+R2

with: Rx=SQR(x̂2+(a+b+z)̂2)

R1=SQR(x̂2+(b+z)̂2)

R2=SQR(x̂2+ẑ2)

DIRECT PHASES: RCV1: Ph1=360a/L+Ph0

RCV2: Ph2=360(a+b)/L+Ph0

-   -   with: L=wavelength and Ph0=constant

REFLECTED PHASES: RCV1: Ph1′=360S1′/L+Ph0

RCV2: Ph2′=360S2′/L+Ph0

PHASE DIFFERENCES: RCV1: d(Ph1)=Ph1′−Ph1

d(Ph1)=360(S1′−a)/L

RCV2: d(Ph2)=Ph2′−Ph2

d(Ph2)=360(S2′−a−b)/L

TOTAL PHASE DIFFERENCE (RCV1 AND RCV2 REFLECTED SIGNALS):d(Ph1)−d(Ph2)=360(S1′−S2′+b)/L

At x=0: S1′=(a+b+z)+(b+z)

S2′=(a+b+z)+z

and: d(Ph1)−d(Ph2)=360(2b)/L

-   -   The reflected signals at RCV1 and RCV2 are in quadrature (90        degrees out of phase) when d(Ph1)−d(Ph2)=90, or when:

b=L/8

In accordance with the invention, the receiving antennas are separatedby a distance b=L/8 to fix the phase difference between the two receiverantennas at 90° (quadrature) when the antennas are directly above thetarget. This provides a means of determining the true amplitude of thecontinuous-wave radiation reflected from the target independent ofinterference effects. In keeping with the constraints discussedpreviously, for b<<z, the amplitudes of the reflected signals at RCV1and RCV2 will he nearly identical, and with 90 degrees phase differencebetween their addition to the direct signals at RCV1 and at RCV2, whichfor b<<a are also nearly identical. Hence, if the direct signals at RCV1and RCV2 are denoted by A1 and A2, with the reflected signals at RCV1and RCV2 by A1′ and A2′, the corresponding total signals T1 and T2 maybe written:

T1=A1+A1′*cos(Phi) and T2=A2+A2′*cos(Phi+90)=A2+A2′*sin(Phi)

If the direct signals A1 and A2 are measured before a target is presentand subtracted from outputs T1 and T2, then the average reflected signalamplitude A′ (which is nearly equal to A1′ and A2′) can be found from:

A′=SQR(T1−A1)̂2+(T2−A2)̂2){tilde over ( )}A1′{tilde over ( )}A2′ since(sin(Phi))̂2)+(cos(Phi))̂2)=1

Thus the amplitude A′ is independent of phase angle Phi (hence withoutinterference effects) when the device is directly above the target(x=0). In the following section, it will be shown that A′ should be amonotonic function of target distance (z) below the lower receiverantenna RCV2 for horizontal distances small compared to verticaldistance (x<<z).

Transmitted and Reflected Signal Strengths

The amplitude A′ described above is a function of the transmitter powerand beam pattern, distance and angle from transmitter to the target,target size, shape and microwave reflectivity, and the distance andangle from the target to the receiving antennas. The following analysisconsiders the strength of the transmitted signal at the target, thereflection by the target, and the reflected signal at the receivingantennas. For simplicity, a cylindrical target of radius Rp is assumedwith reflectivity Q. The transmitted signal strength is assumed to falloff as 1/r, and the antenna pattern for transmitter and receiver is thatof a log periodic Yagi with measured cosine 12 angular falloff. Sincethe length of the cylindrical target illuminated increases with distancein the antenna beam pattern, the strength of the reflected signal willfall off more slowly than 1/r, and an arbitrary (1/r)^(n) is, assumedfor the reflected signal. In the following, the transmitter signal at 1meter along the vertical axis is denoted Ax.

Transmitted Signal Strength at Target:

At=Ax*((cos(Alpha)̂12)/Rx where tan(Alpha)=x/(a+b+z)

Direct Signal Strengths at Receiver:

A1=Ax/a and A2=Ax/(a+b)

Reflected Signal Strength at Receiver:

$\begin{matrix}{{A\; 1^{\prime}} = {Q*{At}*\left( {\left( {\cos \left( {{Beta}\; 1} \right)} \right)\hat{}12} \right)*{\left( {{{Ap}/R}\; 1} \right)\hat{}n}}} \\{{A\; 2^{\prime}} = {Q*{At}*\left( {\left( {\cos \left( {{Beta}\; 2} \right)} \right)\hat{}12} \right)*{\left( {{{Ap}/R}\; 2} \right)\hat{}n}}}\end{matrix}\mspace{11mu} \ldots \mspace{11mu} \frac{{{where}\mspace{14mu} {\tan \left( {{Beta}\; 1} \right)}} = {x/\left( {b + z} \right)}}{{{where}\mspace{14mu} {\tan \left( {{Beta}\; 2} \right)}} = {x/z}}$

A device based an the described design was fabricated and tested. In thetest device, the parameters were: wave freq=2.452897 Ghz (L=12.22 cm.),a=21.6 cm., b=1.528 cm. (L/8), and the exponent n was determined to be0.5. Test results shown in FIG. 6 were obtained for an unburied PVC pipein air with Rp=2.54 cm. The dashed curve is a normalized theoretical fitfrom equations set forth earlier.

Discussion of Results

As shown in the data plot in FIG. 6, the combination of the two receiveroutputs to obtain total reflected signal amplitude results in a signalwhich agrees very well with the theoretical prediction. The averagedeviation of the combined outputs in this plot is approximately 12.5millivolts. This small deviation is due to combined effects ofreflections over the illuminated target length, small differences in theamplitude of the direct and reflected signals at the two receivingantennas, reflections off of nearby objects, and possible, interferenceeffects of wave harmonics. By contrast, the interference effects at thetwo receiver antennas cause deviations as large as ±160 millivolts. Thusthe ⅛ wavelength offset between the two receiving antennas to obtainquadrature in the two receiver phases provides a means of determiningreflected signal amplitude through elimination of the large deviationscaused by interference between the direct signal from the transmitterand the reflected signals. FIG. 7 illustrates that the quadrature outputgreatly reduces the interference pattern variations seen in eitherreceiver output.

The substantial reduction or elimination of interference effects alsoallows the possibility of using two or more pairs of quadraturereceiving antennas, collinear and spaced some distance apart, todetermine target depth. By measuring the true reflected signal amplitudeat each pair, the theoretical curve of signal strength vs. targetdistance may be used to determine target distance independent ofabsolute signal strength. Signal strength may then be used to determinecombined reflectivity and target area, which may be helpful in thedetermination of target characteristics such as composition (metallic ornon-metallic, etc.), size and possibly shape.

Applications

The device described in the previous sections, a combination of acontinuous-wave transmitter and two or more pairs of quadraturereceiving antennas has a wide range of possible application, includingbut not limited to:

(a) detection and location of underground metallic and non-metallicpipes, cables, conduits and utility lines

(b) detection and location of buried metallic and non-metallicmunitions, including mines

(c) detection and location of underground anomalies such as tunnels,shafts or graves.

Tests of the device in an urban environment also indicate that thetransmitted and reflected beams are capable of penetrating buildingwalls constructed of wood, sheetrock, stone or brick, and that thereflections produced by anomalies within or behind the walls may be usedto locate such anomalies. Related applications include:

(a) detection and location of beams, studs, electrical conduit and gasor water pipes within building walls

(b) detection and location of moving objects, including human or animalbodies, behind building walls.

Second Embodiment

In the above-described first embodiment of the invention, quadrature ofthe two receiver phases for reflected radiation eliminates the largedeviations caused by interference between the direct signal from thetransmitter and the two reflected signals. Additional improvement inperformance may be obtained by choice of a geometry in which thetransmitter antenna is shifted to the side of the receiver antennas,with the receiver antennas placed so that both the direct wave from thetransmitter and the reflected wave from a target below the receiverantennas are 90 degrees out of phase at the two receiver antennas(direct and reflected wave quadrature). In addition, the re-positioningof the receiver antennas away from the center of the transmitter antennabeam greatly reduces the direct signal strength at both receiverantennas. A further benefit of this design is that the reflected signalfrom the ground surface is not directly below the receiver antennas, andthe transmitter antenna beam may be tilted toward the receiver antennasso that the center of the transmitter beam illuminates a point at adesired depth below the ground surface.

A simple embodiment of the design discussed above comprises thetransmitter antenna and the upper of the pair of receiver antennasmounted at opposite ends of a portion of a rigid rod or pole which isinclined at an angle (theta) from the vertical, with the second (lower)receiver antenna suspended below the rod or pole by a rigid strut insuch a position that it is ¼ wavelength closer to the ground surfacethan the upper receiver antenna and is also ¼ wavelength closer to thetransmitter antenna than the upper receiver antenna.

The second embodiment of apparatus 28 of the invention using this designis shown in FIGS. 9-12, in which the rod 30 is a ¾-inch aluminum tube(“main beam”). 1-inch square fiberglass tubing mounting posts 32(struts) support the transmitter antenna XKTR and receiver antennas RCV1and RCV2. The transmitter circuit is designated XC. The mounting postsare pinned to the aluminum with ¼-inch fiberglass tubing and glued inplace. Each of the antennas is a directional log periodic type, such asthat shown in FIG. 5 described earlier and is enclosed in an ABS plasticbox 34. Each receiver antenna has a true power detector at the base ofthe antenna on the same circuit board. This detector gives a DC voltageoutput proportional to the detected RF power at the antenna. Thetransmitter antenna has a short coax cable to the transmitter. Thecircuitry will be described later.

As shown in FIG. 9, the rod 30 has a handle 36 intermediate its lengthwhich is held by an operator when the apparatus is in use. Anelectronics unit 38 of the type referred to earlier is mounted on therod between the transmitter and the handle. A battery power supply 40 ismounted on the rod adjacent to the opposite end of the rod and includesbatteries that form a counter-weight for the apparatus.

The geometry (spatial relationships) of such a design is illustrated inFIGS. 13 and 14, with theta, the angle from vertical, set to 45 degrees.This choice of angle was made for comfort of use and balance of thedevice in the user's hands, and is not otherwise prescribed. Thereceiver antennas RCV1 and RCV2 are disposed in parallel (vertically inFIG. 14). In general, the exact dimensions of the device required forboth direct and reflected wave quadrature may be calculated for anarbitrary angle theta and transmitter wavelength L according to thefollowing relationships:

For chosen angle theta and chosen overall distance S1 betweentransmitter antenna and upper receiver antenna, the lower receiverantenna will be supported at a vertical distance D2 below the inclinedrod from a point at a distance k along the rod from the transmitterantenna, with k and D2 calculated from:

k=S1*SQR(1−0.5*L*((1+COS(theta))/(1−(COS(theta))̂2))/S1)

D2=L/4+(S1−k)*COS(theta)

The vertical displacement h of the transmitter antenna above the lowerreceiver antenna is given by:

h=S1*COS(theta)+L/4

and the horizontal distance dx between the two receiver antennas isgiven by:

dx=(S1−k)*SIN(theta)

The angle of tilt of the transmitter antenna from vertical (phi)required for the center of the transmitter beam to cross the verticalmid-line between the two receiver antennas at a distance d below thelower receiver antenna is:

phi=ATN((S1*SIN(theta)−dx/2)/(h+d))

FIG. 14 shows path lengths and phase differences in the secondembodiment, as follows:

DIRECT PATHS: RCV1: S1

RCU2: S2=S1−L/4

REFLECTED PATHS: RCV1: S1′=Rx+R1

RCV2: S2′=Rx+R2

with: Rx=SQR(X2−x)̂2+(h+z)̂2)

R1=SQR(x+dx)̂2+(z+L/4)̂2)

R2=SQR(x̂2+ẑ2)

DIRECT PHASES: RCV1: Ph1=360S1/L+Ph0

RCV2: Ph2=360*S2/L+Ph0

-   -   with: L=wavelength and Ph0=constant

DIRECT PHASE DIFFERENCE: d(Phi)=Ph1−Ph2=90 (QUADRATURE)

REFLECTED PHASES: RCV1: Ph1′=360*S1′/L+Ph0

RCV2: Ph2′=360*S2′/L+Ph0

REFLECTED PHASE DIFFERENCE: d(Phi′)=Ph1′−Ph2′=360*(R1−R2)/L

d(Phi′)=360*(SQR((x+dx)̂2+(z+L/4)̂2)−SQR(x̂2+ẑ2))/L

For z>>dx, at x=−dx/2:

d(Phi′)=90 (QUADRATURE)

Thus in this preferred design, both the direct signals and the reflectedsignals at RCV1 and RCV2 are in quadrature (90 degrees out of phase)when the target is along a vertical line mid-way between the tworeceiver antennas.

As shown, the transmitter antenna is tilted 15 degrees from thevertical, corresponding to the center of the transmitter beam crossingthe vertical mid-line of the two receiver antennas at a point 15 inchesbelow the RCV2 antenna for S1=12.00 inches.

Selection of the various parameters (θ, S₁, d₁, and z_(f)) specifyingthe instrument geometry involves trade-off between instrument size,balance and ease of handling, and expected target burial depth. Thechoices of parameters and factors involved in their selection arediscussed below.

Transmitter-Receiver Distance (S₁) and Angle from Vertical (θ)

Since both receiver antennas RCV1 and RCV2 are always vertical,selection of S₁ for any particular value of θ determines both thehorizontal displacement between the transmitter antenna and the receiverantennas and the height of the transmitter above the ground. Aspreviously discussed, the transmitter height above the ground should besufficient for the transmitted beam to illuminate a spot on the groundwhose area is much larger than that of irregularities on the ground orat shallow depths below it. This produces an average ground backgroundwhich remains relatively constant, since it averages over manyirregularities. However, the transmitter antenna should be far enoughaway from the feet of the user that the user's a feet do not producereflections in the process of walking normally while carrying theinstrument. Likewise, the combination of S₁ and θ should position thetwo receiver antennas RCV1 and RCV2 close enough to the buried target toreceive a strong reflected signal, but high enough above the groundsurface to average over surface irregularities reflections. A naturaldesign scheme places the balance point at the user's hand, with thebatteries providing a rear counter-weight for the weight of the antennasand electronics. A practical limit of 52 inches was chosen for overalllength, and experimenting with various combinations of S₁ and θ for therequired battery and instrument weights led to a selection of S₁=12inches and θ=45° as the optimum configuration for all requirements.

Distance of RCV1 Antenna Below Main Support Beam (d₁)

This distance depends entirely on the physical dimensions of therectangular box enclosing the receiver antenna RCV1 and associated rfdetector. At θ=45°, the minimum possible distance between the mainsupport beam and the center of the receiver antenna RCV1 was d₁=3.5 in.with d₂ calculated to be 8.42 in.

Focus Depth of Transmitter Beam (Z_(f))

The transmitter and receiver antennas beam patterns are identicalsharply focused around the forward direction with power (or sensitivity)dropping to 50% at ±19° about the forward axis. Since the receiverantennas are always vertical in operation, the maximum reflected wavesignal strength at the receiver antennas will be obtained when thetarget is along a vertical line mid-way between the two receiverantennas and is illuminated at the center of the transmitter antennabeam. Thus, for an expected average target burial depth, the transmitterantenna angle from vertical θ may be adjusted to have the antenna beamcenter cross the vertical line between RCV1 and RCV2 at the expectedtarget distance. In the case of mines, the expected burial depth isshallow (3 to 12 inches), whereas in the case of utility lines andpipes, burial may be expected to be deeper (12 to 36 inches). For testpurposes, a value of Z_(f)=15 inches was chosen, resulting in θ=15° andd_(XMT)=4.95 inches, with L₁=13.82 inches and L₂=8.56 inches.

Instrument Set-up and Operation

As shown in FIGS. 15A and 15B, the instrument output consists of ananalog voltage (0-10 v.d.c.) which drives both a voltage controlledoscillator (VCO) and a voltmeter. The output voltage V_(out) isdeveloped from the two receiver r.f. outputs from RCV3 (V1) and RCV2(V2) through a series of amplifiers and summing circuits, as describedbelow:

(A) Both receiver antenna (RCV1 and RCV2) outputs are measured in powerdetectors whose output voltages A1 and A2 are inputs to differenceamplifiers with ×10 gain. The other inputs to the difference amplifiersare voltages from trim pots C₁ and C₂. The outputs from the twodifference amplifiers are thus 10(A₁−C₁) and 10(A₂−C₂).

-   (B) These outputs go to two variable gain amplifiers with adjustable    gain f, whose outputs are 10f(A₁−C₁) and 10f(A₂−C₂).-   (C) These outputs are squared, then summed together and the square    root of the resultant sum obtained. This voltage

V ₀ =SQR[(10f(A ₁ −C ₁))²+(10f(A ₂ −C ₂))²]

which is equivalent to V ₀=10fSQR[(A ₁ −C ₁)²+(A ₂ −C ₂)²]

-   (D) The output V_(o) and an offset voltage C_(o) from a trim pot are    inputs to a difference amplifier whose output is V_(o)−C_(o).-   (E) The output V_(o)−C_(o) is then input to a variable gain    amplifier whose gain g is set to: g=10/(10−C_(o)) which sets the    output to the meter and VCO so that their full range is scaled    (0-10 v) as V_(o) varies from C_(o) to ZC_(o).    The instrument is set up for operation in the following steps:

1. Calibration for Direct Signals at RCV1 and RCV2

With the antennas pointed upward-at the sky,

(a) the meter input is switched to the output developed in step (B) forRCV1, which is 10f(A₁−C₁) and the voltage C₁ is adjusted in the variablepot until the meter reading is 0 (i.e., C₁=A₁). This pot setting is heldfor RCV1.

(b) meter input is switched to output for RCV2, which is 10f(A₂−C₂) Potvoltage C₂ is adjusted until meter reading is 0 (C₂=A₂) and pot settingis held for RCV2.

(c) Meter input is switched to the output from step (D), which isV_(o)−C_(o).

2. Neutralization of Earth Background

With the device held in normal operating position, with antennas pointedtoward the ground in a location assumed to have no nearby buriedtargets,

(a) set pot adjusting voltage C_(o) to mid-range

(b) adjust variable gain f until meter reading is zero

(c) start research for buried objects, adjusting pot controlling C_(o)as squelch control to compensate for changes in earth background withterrain changes (gravel, bare earth, grass, etc.)

While preferred embodiments of the invention have been shown anddescribed, it will be apparent that changes can be made withoutdeparting from the principles and spirit of the invention, the scope ofwhich is defined in the accompanying claims.

1. Apparatus for detecting a target hidden behind a surface, comprising:a transmitter that transmits a beam of continuous-wave radiation; afirst receiver disposed to receive such radiation reflected from atarget; and a second receiver disposed to receive such radiationreflected from the target, wherein the receivers are spaced from thetransmitter and are spaced from one another according to predeterminedspatial relationships such that the phase of reflected radiationreceived by the first receiver is in quadrature with the phase ofreflective radiation received by the second receiver, and wherein theapparatus further comprises circuitry that combines signals derived fromthe receivers to produce an output.
 2. Apparatus according to claim 1,wherein the output corresponds to the square root of the sum of twosquared signals derived from the receivers.
 3. Apparatus according toclaim 1, wherein the spatial relationships are such that the receiversreceive direct radiation from the transmitter before radiation reachesthe target.
 4. Apparatus according to claim 3, wherein the phase ofdirect radiation received by the first receiver is in quadrature withthe direct radiation received by the second receiver.
 5. Apparatusaccording to claim 1, wherein the radiation is microwave radiation andeach of the transmitter and the receivers includes an antenna with adirectional beam pattern.
 6. Apparatus according to claim 5, wherein thespatial relationships are such that the beam patterns extend insubstantially the same direction.
 7. Apparatus according to claim 6,wherein the microwave radiation has a predetermined frequency and thereceivers are separated by a distance L/8, where L is the wavelength ofthe radiation.
 8. Apparatus according to claim 5, wherein the beampatterns of the receivers are substantially parallel and the beampattern of the transmitter is inclined with respect to the beam patternsof the receivers.
 9. Apparatus according to claim 8, wherein themicrowave radiation has a predetermined frequency, and the apparatus isconstructed such that, in use, one of the receivers is a quarterwavelength closer to the surface than the other receiver.
 10. Apparatusaccording to claim 5, wherein the transmitter and the receivers aremounted on an elongated support with the receivers adjacent to an end ofthe support and the transmitter spaced from the end of the support. 11.Apparatus according to claim 10, wherein the support comprises a rod,the transmitter and the first receiver are mounted on a same side of therod and the second receiver is mounted on an opposite side of the rod,and both receivers are the same distance from the transmitter. 12.Apparatus according to claim 11, wherein the apparatus is constructedsuch that, in use, the rod is oriented substantially perpendicularly tothe surface, with the end of the rod adjacent to the surface. 13.Apparatus according to claim 12, wherein the rod has a handleconstructed to permit an operator to hold the rod oriented substantiallyperpendicularly to the surface.
 14. Apparatus according to claim 13,wherein the rod has an electronics unit mounted thereon that includes asource of radiation energy, a readout device, a power supply, andcontrols.
 15. Apparatus according to claim 5, wherein the transmitterand the receivers are supported on an elongated rod with the receiversadjacent to an end of the rod and the transmitter spaced from the end ofthe rod, the transmitter and the receivers are disposed at a same sideof the rod, and the apparatus is constructed such that, in use, thereceivers are adjacent to the surface, the transmitter is remote fromthe surface, and the rod is inclined to the surface.
 16. Apparatusaccording to claim 15, wherein the rod has a handle and has anelectronics unit thereon between the handle and the transmitter. 17.Apparatus according to claim 16, wherein the rod has a counter-weightadjacent to its opposite end.
 18. Apparatus according to claim 17,wherein the electronics unit has a source of radiation energy, a readoutdevice, and controls, and the counter-weight is part of a power supplyfor the apparatus. 19-26. (canceled)